1. Field of the Invention
The present invention relates to a hot press which is used in production of multilayered substrate and, more particularly, to a hot press which is suitable in forming a multilayered substrate, such as substrate of a printed circuit board or a ceramics board, having high degree of density and quality as well as reduced thickness, and which can reduce the production cost.
2. Description of the Related Art
Densifying a multilayered substrate essentially requires prevention of oxidation at the junctions and removal of voids between the layers of the multilayered substrate. To cope with such demands, a method has been proposed in, for example, Japanese Unexamined Patent Publication No. 62-156931, in which blanks of layers are enclosed in an enclosure made of a film or a sheet and are pressed by heat plates after the interior of the enclosure is evacuated.
Meanwhile, Japanese Unexamined Patent Publication No.3-128195 discloses an art in which a heat insulating plate is placed between heat plates for heating blanks of a multilayered substrate and bolsters which press the blanks through the heat plates, so as to uniformly apply heat to the blanks of the multilayered substrate. The space between the bolsters is enclosed by sealing means and is evacuated until the adhesive is softened and fluidized, whereas, during the setting, air pressure is applied to maintain a pressurized atmosphere in the space, thereby attaining uniform pressure distribution during bonding.
The bonding pressure applied during bonding is determined by dividing the area of the substrate surface to be bonded by the load applied by a main ram, when the bonding pressure is applied in atmospheric pressure. However, when the pressure in the chamber enclosing the heat plates has been reduced, the load applied by the main ram is increased by an amount corresponding to the product of the overall area of the space inside the chamber and the amount of reduction in the pressure. Thus, the bonding pressure, i.e., the substrate surface pressure, is increased. Conversely, when the space inside the chamber enclosing the heat plates is pressurized, the load applied by the main ram is reduced by an amount which is the product of the overall area of the space inside the chamber and the amount of increase in the pressure,thus reducing the bonding pressure.
The amount of dimensional change in the multilayered substrate after the bonding is largely influenced by the bonding pressure. Therefore, in a low-pressure forming process in which positive air pressure is applied to achieve a high precision of forming, it is necessary to exactly detect even a small variation in the bonding pressure and to feed such a small change back to the hydraulic pressure on the ram, otherwise the forming at the desired pressure is failed to impair the precision of formation of the product multilayered substrate.
Bonding by a conventional hydraulic pressure control system under application of air pressure tends to cause a bonding failure due to a significant reduction in the bonding pressure caused by a pneumatic reacting force or by a lowering of the bolster.